Aging and muscle fiber type alter K+ channel contributions to the myogenic response in skeletal muscle arterioles

Abstract

Aging diminishes myogenic tone in arterioles from skeletal muscle. Recent evidence indicates that both large-conductance Ca2+-activated (BKCa) and voltage-dependent (KV) K+ channels mediate negative feedback control of the myogenic response. Thus we tested the hypothesis that aging increases the contributions of KV and BKCa channels to myogenic regulation of vascular tone. Because myogenic responsiveness differs between oxidative and glycolytic muscles, we predicted that KV and BKCa channel contributions to myogenic responsiveness vary with fiber type. Myogenic responses of first-order arterioles from the gastrocnemius and soleus muscles of 4- and 24-mo-old Fischer 344 rats were evaluated in the presence and absence of 4-aminopyridine (5 mM) or iberiotoxin (30 nM), inhibitors of KV and BKCa, respectively. 4-Aminopyridine enhanced myogenic tone with aging and normalized age-related differences in both muscle types. By contrast, iberiotoxin eliminated age-related differences in soleus arterioles and had no effect in gastrocnemius vessels. KV1.5 is an integral component of KV channels in vascular smooth muscle; therefore, we determined the relative protein expression of KV1.5, as well as BKCa, in soleus and gastrocnemius arterioles. Immunoblot analysis revealed no differences in KV1.5 protein with aging or between variant fiber types, whereas BKCa protein levels declined with age in arterioles from both muscle groups. Collectively, these results suggest that the contribution of BKCa to myogenic regulation of vascular tone changes with age in soleus muscle arterioles, whereas increased KV channel expression and negative feedback regulation of myogenic tone increases with advancing age in arterioles from both oxidative and glycolytic muscles.

Fig. 1.

Diameter changes in response to increasing intraluminal pressure in soleus (A) and gastrocnemius (B) muscle arterioles. In both muscle fiber types, arterioles from young animals exhibited more robust myogenic constriction than arterioles from old animals. C and D: resting myogenic tone in the same soleus (C) and gastrocnemius (D) arterioles from A and B, respectively, at increasing intraluminal pressures. Young soleus, n = 10; old soleus, n = 12; young gastrocnemius, n = 10; old gastrocnemius, n = 9. Values are means ± SE. *P < 0.05.

Fig. 2.

Concentration-response relationship to 4-aminopyridine (4-AP) in soleus and gastrocnemius muscle arterioles from young and old Fischer 344 rats. 4-AP induced greater constriction in soleus muscle arterioles from old compared with those from young rats (A). Aging had no effect on responsiveness to 4-AP in gastrocnemius muscle arterioles (B). Young soleus, n = 8; old soleus, n = 7; young and old gastrocnemius, n = 10. Values are means ± SE. *P < 0.05.

Fig. 3.

Myogenic responsiveness in control and 4-AP-treated arterioles from soleus and gastrocnemius muscles. Voltage-dependent K⁺ (K_V) channel blockade by 4-AP (5 mM) elicited greater myogenic constriction in arterioles from old animals compared with those from young animals in both muscle fiber types (A and B). 4-AP eliminated age-induced differences in myogenic responsiveness in first-order (1A) arterioles from both soleus (A and C) and gastrocnemius (B and D) muscles. For 4-AP-treated groups: young soleus, n = 13; old soleus, n = 8; young gastrocnemius, n = 10; old gastrocnemius, n = 7. Values are means ± SE. *P < 0.05, 4-AP treatment vs. age-matched control.

Fig. 4.

Concentration-response relationship to iberiotoxin (IBX) in soleus and gastrocnemius muscle arterioles from young and old rats. IBX-induced constriction was greater in arterioles from old compared with those from young rats (A). No age-related differences in sensitivity to IBX were detected in arterioles from gastrocnemius muscle (B). Values are means ± SE. Young gastrocnemius, n = 6; all other groups, n = 5. *P < 0.05.

Fig. 5.

Myogenic responses of control and IBX-treated soleus and gastrocnemius muscle arterioles. Large-conductance Ca²⁺-activated K⁺ (BK_Ca) channel blockade by IBX significantly enhanced myogenic constriction in soleus muscle arterioles from old rats (n = 3) while having no effect in arterioles from young rats (n = 6; A). IBX did not alter myogenic responsiveness in gastrocnemius muscle arterioles from young (n = 6) or old (n = 4) rats (B). Age-induced differences in resting myogenic tone were eliminated in soleus muscle arterioles (C) but maintained in gastrocnemius muscle arterioles (D) following IBX treatment. Values are means ± SE. *P < 0.05, IBX treatment vs. old control. #P < 0.05, significantly different from young IBX.

Fig. 6.

Effect of fiber type on constrictor responses to 4-AP and IBX. Arterioles from soleus muscle exhibited greater constriction to 4-AP compared with those from gastrocnemius muscle in both young (A) and old rats (B). IBX had similar effects in arterioles from soleus and gastrocnemius muscles of young animals (C) but elicited greater constriction in soleus vs. gastrocnemius muscle arterioles in old rats (D). Values are means ± SE. *P < 0.05.

Fig. 7.

Immunoblot analysis of K_V1.5 (A) and BK_Ca (B) protein content normalized to GAPDH in soleus and gastrocnemius muscle arterioles. K_V1.5 levels did not change with age or fiber type. BK_Ca protein levels declined with age in arterioles from both soleus and gastrocnemius muscles. Young soleus, n = 5; old soleus, n = 4; young and old gastrocnemius, n = 5. Values are means ± SE. *P < 0.05.